A retroreflector is an optical device used to reflect radiation beams from an associated radiation source back to a radiation sensor positioned at the source along paths substantially parallel to those of the corresponding incident beams. Known retroreflectors include cube corners, cat's eyes mirror systems, and embedded lens systems. Retroreflectors can be advantageously incorporated into a large variety of devices, e.g., devices used for determining the position of objects such as satellites, planes and ships; optical communication devices; and road signs and traffic markings.
Problems typically encountered with known retroreflectors include a small field of regard and inconsistent return efficiency over the field of regard. The field of regard is the portion of a retroreflector in which the retroreflector can receive radiation and is capable of reflecting a sufficient amount of such radiation back along substantially parallel paths to permit sensing. Typically it is a cone shaped sector that has its apex located within the retroreflector and is symmetrical to the axis of symmetry for the retroreflector. A field of regard is expressed in terms of the number of degrees of the corresponding angle at the field's apex and is measured transversely through the retroreflector axis. Therefore, for a retroreflector with a nearly hemispherical field of regard, the angle would approach 180.degree..
The return efficiency of a retroreflector is an indication of the amount of radiation from a source which is reflected back to the source by the retroreflector, and can be evaluated through the use of a radiation detection means located at the source. For the present purposes, radiation which is not reflected back to the source is disregarded when calculating return efficiency. In other words, radiation which is not retroreflected along a path substantially parallel to the incident beam is disregarded. A retroreflector with a consistent return efficiency will reflect a substantially constant proportion of incident radiation back towards the source irrespective of the orientation of the retroreflector to the source, provided that the incident radiation falls within the field of regard.
Cube corner retroreflectors are well known in the art and are disclosed, for example, in U.S. Pat. No. 4,143,263 by Eichweber, issued Mar. 6, 1979. Cube corners comprise three flat reflecting surfaces arranged at 90.degree. angles to each other, with the interior surfaces producing a reflected beam which is parallel to the incident beam. One problem with cube corners is that the field of regard is typically limited to about 60.degree.. Another problem encountered with cube corners is their typically inconsistent return efficiency. As the unit is tilted relative to the incident beam, the radiation collected by the unit is decreased by the cosine of the tilt angle, causing the total radiation returned to be reduced by the same proportion. In addition, as the unit is tilted, the return beam is spread by diffraction, again in proportion to the cosine of the tilt angle. The result is a decrease in retroreflected radiation detected at the radiation source proportional to the square of the cosine of the tilt angle of the retroreflecting unit. Additionally, because there are six different sequences for the three reflections used to produce the return beam, a mechanical misalignment of any single surface will produce a group of six return beams rather than a single beam.
A cat's eye retroreflecting system, as disclosed by Connes et al., Journal of the Optical Society of American, Vol. 56, No. 7, p. 896 (1966), consists of a concentric pair of spherical mirrors, the first of which is concave and produces a focus spot of a radiation source on a convex reflecting surface of a second mirror. This type of system is relatively large, has a small field of regard and is sensitive to mechanical misalignments and thermal deviations.
A common type of retroreflector system utilized on traffic signs and for road markings is known as an "embedded lens system." Such retroreflector systems are disclosed, for example, in U.S. Pat. No. 3,889,027 by White, issued June 10, 1975; U.S. Pat. No. 4,192,576 by Tung et al, issued Mar. 11, 1980; U.S. Pat. No. 4,708,920 by Orensteen et al., issued Nov. 24, 1987; and U.S. Pat. No. 4,511,210 by Tung et al., issued Apr. 16, 1985. Embedded lens retroreflector systems typically comprise transparent spheres or microspheres partially embedded in a reflective layer.
One problem with such systems is that they typically have a low field of regard. For example, it is noted in U.S. Pat. No. 4,511,210 that in embedded lens systems known prior to the disclosure in U.S. Pat. No. 4,511,210, the retroreflector return efficiency generally declines to less than three-quarters of its maximum value outside of a field of regard of about 70.degree. (called the "three-quarter brightness angle"). The patent then discloses a system which retains three-quarter brightness within a field of regard equal to about 80.degree. (See column 2, lines 49-57).
Another problem encountered with embedded lens systems is undesirably high aberration. This aberration is partially due to the small size of the lenses employed in embedded lens systems. As is disclosed in U.S. Pat. No. 3,889,027, the Federal specifications for retroreflective surfaces limit the thickness of the entire composite to less than 0.010 inch (See column 2, lines 18-36). As a result, the lenses embedded within this 0.010 inch layer must be extremely small.
As is apparent from the above, current retroreflectors suffer from a number of problems. For example, some current retroreflectors suffer a loss in return efficiency as a function of incident angle, usually proportional to the square of the cosine of the angle between the incident beam and the axis of symmetry for the retroreflector. This can be a severe restriction on the allowable orientation of the retroreflector relative to a radiation source. Additionally, current retroreflectors such as cube corners are sensitive to manufacturing errors, which for example may result in multiple, non-parallel return beams for a single incident beam. Furthermore, current retroreflecting systems such as cat's eye mirror systems are expensive, occupy a relatively large volume, are very sensitive to alignment errors introduced by manufacturing errors or thermal changes, and have a restricted field of regard. Embedded lens systems also typically have a limited field of regard and an undesirably high amount of aberration. Therefore, a retroreflector which provides the design flexibility required to compensate for the foregoing deficiencies would be advantageous.
One specific type of aberration which is often encountered with embedded lens and other lens systems is known as chromatic aberration. The indistinct color effects observed along the edges of images formed by a simple lens constitute what is known as the chromatic aberration of the lens. This aberration is due to the fact that the glass, or any other transparent substance out of which a lens may be constructed, disperses incident radiation (i.e., refracts light of different wavelengths by different amounts). The image formed by a concave, metal or silverbacked glass mirror is free from chromatic aberration since all incident radiation is reflected in the same direction. Therefore, retroreflectors which only employ mirrors, i.e., cube corners and cat's eye mirror systems, do not suffer from chromatic aberration while those which employ lenses, i.e., embedded lens systems, may suffer from chromatic aberration.
Another type of aberration which is encountered in both lens and mirror systems is spherical aberration. If the refracting surface of a lens or the reflecting surface of a mirror are spherical, the rays refracted through or reflected from the outer portions of these surfaces, as defined by reference to a center path between the radiation source and center of curvature for the surfaces, will be brought to a focus in a different plane than those rays on the center path, thus producing a blurring of the resultant image known as spherical aberration. This effect is more pronounced in short-focus lenses (e.g. embedded lens systems) and mirrors than in long-focus instruments, because the curvature of the surfaces of the short-focus instruments is greater. Because all of the retroreflectors described above incorporate lenses and/or mirrors, spherical aberration may be a problem if any of their surfaces are spherical. Therefore, it would be advantageous to produce a retroreflector in which the problems of chromatic and spherical aberration may be compensated for.
Not only is it useful to return reflected radiation beams along paths substantially parallel to the corresponding incident radiation beams, but it is also desirable in certain instances to modulate the return radiation beams. Modulation of retroreflective radiation is disclosed in, for example, U.S. Pat. No. 4,143,263 by Eichweber, issued Mar. 6, 1979; U.S. Pat. No. 4,134,008 by de Corlieu et al., issued Jan. 9, 1979; U.S. Pat. No. 3,989,942 by Waddoups, issued Nov. 2, 1976; U.S. Pat. No. 3,863,064 by Doyle et al, issued Jan. 28, 1975; and U.S. Pat. No. 4,361,911 by Buser et al., issued Nov. 30, 1982. However, because the systems disclosed in the above-mentioned patents employ retroreflectors generally of the types described above, they suffer from the same disadvantages noted above. Therefore, it would be advantageous to produce a modulated retroreflector which does not suffer from the disadvantages discussed hereinbefore.
In summary, it would be desirable to provide a retroreflector in which one or more of the following advantages are obtainable: (1) A retroreflector with a field of regard approaching 180 .degree.; (2) A retroreflector with substantially consistent return efficiency as a function of incident beam angle relative to the retroreflector orientation within its field of regard; (3) A retroreflector with simple, rugged construction and relative intolerance to manufacturing errors and thermal effects; (4) A compact retroreflector; (5) A retroreflector with high retroreflector efficiency over a wide spectral band; (6) A retroreflector with minimal spherical aberration; and (7) A modulating retroreflector.